When spider webs were left intact in the aluminum-glass boxes in the laboratory, most animals ate old webs daily and built new ones. The new webs were built with a larger amount of thread protein, increasing steadily in weight with increasing length of the non-elimination period, to more than 3 times in 8 days. The increased amount of thread material was first used to build larger webs with a longer thread, and later larger webs with thicker threads. A change in the number of spiral turns always went together with a change in web size and/or the number of radii thus confirming PETER'S segment rule. Experiments with radioactive labeled webs indicated that some material for the new web came probably through ingestion and reexcretion of the old web's protein. But experiments with intact versus destroyed and non-eliminated webs showed that probably not only old web material but also the intact web structure were clues for larger new webs. The deterioration of silk as well as continuous new thread synthesis in the spinning glands are suspected as reasons for the frequent rebuilding of webs.